Conventional inks for lithographic printing are formulated with vegetable or mineral oils, alkyd resins, and phenolic resins or hydrocarbon resins. On porous substrates, such as paper or board, low viscosity, low aromatic mineral oils are used. The mineral oils penetrate into the substrate to induce physical drying, or setting (“quick set” effect). However, the quick set effect cannot be employed when printing on non-porous substrates, such as plastic films. For printing on non-porous substrates, inks are formulated using vegetable oils and their esters (known as drying oils), to dissolve the hard resin and for reducing viscosity. The oils in lithographic inks participate in the drying of the inks by an oxidation reaction.
A major disadvantage of conventional lithographic inks is the slow drying speed. Conventional lithographic inks based on oils and alkyds dry slowly by penetration of the substrate (setting), and oxidation. This is especially critical for non-porous substrates where an oil does not penetrate. This negatively impacts the productivity a converter can achieve. Thus, a need exists for producing faster drying inks to improve productivity.
Energy curable inks have also been employed in lithographic printing. Energy curable inks have improved gloss and resistance properties. However, there are drawbacks to using energy curable inks for lithographic printing. For example, pigment wetting of energy curable inks is not as good as for conventional inks, resulting in print density problems and the like. In addition, energy curable inks show problems with low shear viscosity, less stable emulsion with water, and higher tack. The lack of stability in an emulsion with water results in a smaller water balance on the printing press. Higher tack may lead to picking of fibers or coating when printing on paper or board.
Depending on the requirements of a particular print job, a converter may want to use both conventional and energy curable lithographic inks. Conventional inks are generally printed with N-buna-nitrile rubber (NBR) rollers, while energy curable inks are generally printed with ethylene-propylene-diene-monomer (EPDM) rollers. Energy curable inks are not compatible with NBR rollers, and conventional inks are not compatible with EPDM rollers. Use of an incompatible ink leads to roller swelling, thus compromising print quality. To employ both conventional and energy curable inks on the same press, a converter must switch rubber rollers and blankets on the printing press.
Often, to improve gloss of prints made with conventional inks, a converter will apply an energy curable topcoat over the printed substrate. But, the energy curable topcoat is not compatible with the oil and alkyd based inks, and you often get “gloss back”. Gloss back is the phenomenon where a radiation curable coating applied over a conventional ink loses gloss within a short period after cure, typically within a day. To improve gloss back, it is generally necessary to apply a water-based primer between the print and the energy curable topcoat.
In an attempt to overcome these problems, there has been an effort to develop “hybrid” inks which comprise both conventional ink components and energy curable components. Hybrid inks are described by Paul Gaevert at Radtech Conference Nov. 3-5, 2003, Conference Proceedings “Ink performance properties of UV, conventional and hybrid sheet-fed inks;” Tony Bean in “Radtech Report Oct. 2009, Hybrid Sheetfed lithographic systems—State of the Art;” and Dieter Kleeberg in “Quality enhancement with hybrid production” in Process 2006 (publication of press maker KBA).
Radiation curable hybrid inks are radiation curable inks which also contain raw materials from conventional inks, such as oils, alkyd resins, and hard resins. Hybrid inks combine different drying properties. They preferably dry under UV (ultraviolet) or EB (electron beam) radiation, and also dry by oxidation by air or heat drying. Moreover, the inks can dry by penetration of the oils into the substrate like in oil-based conventional inks. One advantage that may be observed with hybrid inks is that they can be directly overprinted with a radiation curable coating with only minor loss in gloss (i.e. minor gloss back). Consequently, an aqueous primer between ink and coating, as well as a double coater on press, can be spared.
However, because radiation curable hybrid inks are based on chemically different materials, such as non-polar vegetable oils or minerals, and phenolic or hydrocarbon resins, in combination with the more polar monomeric acrylates and resins, good compatibility is not easy to achieve.
One drawback of hybrid inks is that the stability of the hybrid ink has to be balanced. In the presence of oxygen, the oil-based materials can start to build up viscosity by oxidation, especially in the presence of a dryer, whereas, on the other hand, oxygen stabilizes radiation curable components such as acrylates.
The major disadvantage of currently available hybrid inks is due to the incorporation of oils and alkyds, which dry slowly by penetration (setting) and oxidation, adversely affecting the drying speed of the entire ink. As the determining factor for ink drying is now the setting of the oil and the oxidation of the alkyd, a converter is usually not getting the productivity (line speed) of a 100% radiation curable ink. This is especially critical for non-porous substrates where an oil does not penetrate.
It can therefore be seen that the demands required from the formulation of hybrid inks is enormously complex, and optimizing the properties of inks is equally complex. From a lithographic standpoint, it is advantageous to have an ink with a high amount of oils, alkyds, and hard resin. Conversely, in view of drying speed, productivity and gloss back, a high amount of radiation curable monomers and oligomers is favorable.